Various apparatuses and methods have been developed for brazing and soldering together the elements of a larger structure. In some instances, the braze or solder material first is applied in solid form at the contacting parts of the elements. Then the assembled structure is placed in a suitable furnace where the temperature is raised to the fusion temperature of the braze or solder material and then lowered until the material fuses again to join the structural elements. In such familiar applications, both fluxed and fluxless joining materials have been used, in accordance with the characteristics of the furnace and the desired properties of the completed structure.
For example, Kendziora et al. U.S. Pat. No. 3,882,596 discloses an apparatus for fluxless soldering aluminum-containing workpieces in a controlled atmosphere. A complex continuous belt furnace with air locks at either end heats the workpieces by means of electrical heaters within the furnace. Additional heat is added by a separately preheated, controlled atmosphere of nitrogen or argon which flows into the furnace through conduits which may be of copper. Typically, such belt furnaces heat up quickly and have low cycle times, but their temperatures and the contents of their atmospheres are hard to control. More recently, Wagner U.S. Pat. Nos. 4,140,266 and 4,231,508 disclose an apparatus for fluxlessly soldering aluminum plates of a heat exchanger. An inert gas such as nitrogen is preheated to a temperature near the soldering temperature and then flowed through the stack of plates to achieve sufficient heating for soldering.
In the manufacture of today's high density electronic devices, integrated circuit chips have been attached to supporting substrates using both fluxed and fluxless solders. Various fluxes and various ambient atmospheres, including nitrogen, forming gas (90% nitrogen and 10% hydrogen) and hydrogen, have been used in continuous belt furnaces for such chip joining. However, when single chips are joined using flux to a multi-chip module, multiple reflows of the solder joints will typically occur, causing the solder joints to degrade and resulting in increased resistance and decreased strength.
As reported by D. A. Chance in IBM Technical Disclosure Bulletin Vol. 23, No. 7A, page 2990, of December, 1980, reflow solder joints can be made without flux in a properly controlled atmosphere. Chance found that the forming gas used in earlier chip joining processes was often contaminated with other gases and vapors including oxygen and water. To remove the oxygen, Chance used a quartz tube heated to 540.degree. C. and filled with platinum wires, through which the forming gas was flowed prior to introduction into the chip joining furnace. Low partial pressures of oxygen resulted due to the catalytic effect of the hot platinum, which caused the oxygen to react with hydrogen to form water. In the chip joining furnace, the chips were supported on their substrates which rested on a copper platen heated by an infrared heater. A rather large furnace chamber surrounded the chips and substrates, apparently requiring the use of a vacuum system to remove the air in the chamber at the start of the joining process.
While such prior art methods and apparatuses have achieved a measure of success for their specific applications, problems have continued to exist when electronic chips are to be bonded to substrates. Although rapid heating of the chip and substrate has been possible in many cases, maintenance of a relatively uniform temperature distribution across the substrate and chips has been difficult to achieve. Such uniformity is important in many applications to ensure that the solder materials are not heated so far beyond their fusion temperature that they are degraded during reflow cycles. For example, in the system disclosed by Chance, the infrared heaters depend upon adequate thermal conductivity to heat the chips and substrates. While Chance's copper platen helps in this regard, the poor conductivity of the substrate interferes with uniform delivery of heat to the site of the chips where the solder reflow must take place. In addition, the gas used by Chance apparently enters the furnace at an uncontrolled temperature; and the furnace has a thin cover which would cause uncontrolled heat loss.
Thus, a need has continued to exist for a method and apparatus for heating objects, such as electronic chips to be joined to substrates, in which not only low oxygen content and fast heating rates, but also fast heat-up/cool-down cycles and essentially uniform temperature distributions are achievable.